This application is the National Phase of International Application PCT/NL01/00035 filed Jan. 18, 2001 which designated the U.S. and which International Application was published under PCT Article 21(2) in English.
1. Field of the Invention
The invention relates to a jack-up platform comprising a deck structure with floating power and a column with a base structure, in which a jack-up system acts on the column so as to be able to move and secure the deck structure and the column relative to each other, and the base structure comprises a base and at least three separate base supporting parts. The invention also relates to a method for installing such a jack-up platform.
2. Background of Related Art
Such a jack-up platform is known from WO-A-81/000423. Initially, such jack-up platforms were used in relatively shallow water, but in the course of time the use also shifted to increasingly deeper water, for which water depths of 300 meters and more may be considered. Even under these circumstances the jack-up platform must be stable and relatively rigid under all kinds of loads, for instance as a result of water flows and wind. On the other hand, for transport, handling and movement the structure must preferably be of light design. To meet these outwardly incompatible requirements, the structure, after installation on a sea floor, may be stiffened by means of guylines, as, for instance, known from U.S. Pat. No. 4,378,178. The provision of such guylines, however, is an extremely difficult and labor-intensive, if not, in particular in great depths, practically impossible activity.
The object of the invention is to provide a jack-up platform which meets the above incompatible requirements but which, at the same time, is relatively rapid and simple to install and move, even in great water depths.
According to the invention this object is achieved for a jack-up platform of the type described in the opening paragraph if the deck structure, through tensioning means starting from at least three spaced coupling places, can be connected with at least three mutually spaced connecting places on or near the base supporting parts. By taking these measures, a relatively light structure becomes possible, while the stability of the jack-up platform remains ensured through, on the one hand, the base structure with a base and a plurality of base supporting parts forming a multipoint support and, on the other hand, the stiffening of the whole jack-up platform in the operative position through the tensioning means between the coupling places and the connecting places, with at least one tensioning means extending between each coupling place and each connecting place. For this a kind of stay structure may be considered, with one or a plurality of tensioning means extending between each connecting place and each coupling place and/or tensioning means extending from a coupling or connecting place to more than one connecting or coupling place. A sufficiently rigid whole, however, can already be realized if each connecting place is connected with a coupling place through one tensioning means, while these tensioning means preferably extend vertically, which results in a structure to be realized in a relatively simple manner.
Since a body with a three-point support can essentially be installed so as to be completely stable, it is preferred according to a further embodiment of the invention that three base supporting parts are provided, which are substantially symmetrically arranged around the column, which extends through an opening centrally provided in the deck structure.
The tensioning means can be realized in many ways, for which a tensioning means may be considered which has the form of an elongated element consisting of at least one pipe, rod, chain, cable or similar element. It is also possible that a tensioning means has the form of a composite lattice body, which, for that matter, can be of substantially lighter design than the conventional lattice supporting columns in the conventional known jack-up platforms, because such a tensioning means column has no supporting function and is under tensile stress, instead of compression stress.
To increase the floating power of the jack-up platform when towing it to the desired location, it may be provided that at least the base or at least one of the base supporting parts comprises a body with floating power, such as a tank. In the operative position of the jack-up platform such a body can further be advantageously used for all kinds of storage and transshipment purposes.
Also, according to a further embodiment of the invention, it may be preferred that the base structure can be pulled up into a suitable hole provided in the deck structure, which can reduce the traveling resistance during the conveyance of the jack-up platform to the installation site. In this case it will further often be preferred that the base structure is vertically of relatively flat design so that the height of the deck structure, in its totality, can also remain relatively low.
If according to a further embodiment of the invention the deck structure is composed of a lower part and an upper part, the upper part receding relative to the lower part over at least part of the circumference, the jack-up platform can be partly sunk in the manner of a semi-submersible, that is to say sunk so far that the broader lower part is under the water surface and the narrower upper part at least partly above. Thus the jack-up platform can be made more independent of wave motion when lowering the column and bringing the base structure into contact with the floor. This effect can further be optimized if the deck structure consists of an edge body surrounding a central through hole, in which hole the jack-up system is centrally and locally installed. It is further preferred that the receding part of the upper part adjoins the central through hole. Over the entire height of the deck structure the exterior of the jack-up platform may have a form adjusted to a minimum traveling resistance.
To obtain a proper anchoring of the column to the floor, it may be preferred, depending on the nature of the floor, that near the base supporting parts an apron-shaped extension is provided, which can penetrate into the sea floor.
Under the circumstances it will not always be possible that the base supporting parts are supported on a flat horizontal surface. In this connection it is preferred according to a further embodiment of the invention that means are provided to apply a filling material, such as grout, under a base supporting part so that by supplying such a filling material near at least one of the base supporting parts the column can always be pressed into a purely vertical position and be aligned. As counterpart thereof or in addition thereto it is also possible to provide means for creating a reduced pressure under a base supporting part.
The invention also relates to a method for installing a jack-up platform as discussed above. To this end, it is proposed according to the invention that the platform, floating in the water with a pulled-up column, is brought to the desired location where the column with its base supporting parts is brought into supporting contact with the floor under water, the deck structure is raised along the column so as to be out of contact with the water, the support of the base supporting parts on the floor is completed while vertically aligning the column, each connecting place on or near the base supporting parts is connected through a tensioning means with at least one coupling place on the deck structure, the tensioning means are tensioned after bringing the deck structure to working height, and the deck structure is secured relative to the column.
To be able to carry out the provision of the tensioning means in a relatively simple manner, that is to say without many underwater operations, in particular in greater depths, it may be preferred according to a further embodiment of the invention that the tensioning means are fixed from the deck structure in the connecting places before the column is brought downward and, via the base structure, into supporting contact with the floor. If desired, the tensioning means may also be continuously fixed in the connecting places.
If the platform is provided with a deck structure with a lower part and an upper part receding at least partly relative to the lower part, it may be realized according to a further embodiment of the invention that before lowering the column the deck structure is sunk until the broader lower part is under the water surface. It is thus possible by sinking the jack-up platform partly in the manner of a submersible to lower the column and to install the base structure on the floor while being less dependent on wave motion.
Depending on the circumstances, it may further be preferred from a viewpoint of stability that the installation method is supplemented with an additional intermediate step in the sense that, after installing the base structure on the floor, completion of the support between the base supporting parts and the floor and vertical alignment of the column take place when the deck structure is raised to a relatively low height above the water surface, after which the deck structure is raised to the desired working height before the tensioning means are subjected to tension.
The tensioning of the tensioning means can take place in many well-known ways. According to a further embodiment of the invention, however, it is preferred that the available jack-up system is advantageously used, which is simply possible if the tensioning means are subjected to tension by further raising the deck structure over a predetermined distance.
To keep the required navigable depth for the jack-up platform as low as possible during transport to the site where it needs to be installed, it may further be preferred that, when bringing the platform to the desired location, the column is pulled up so far that the base structure is in the immediate vicinity of the floating deck structure, which position can be realized with the jack-up system in a simple manner. To even further reduce the water resistance experienced during traveling, it is preferred that the column is pulled up so far that the base structure is completely within a space provided in the deck structure for this purpose.